I find the difference between Reddit and HN is the quality of moderation. HN is fantastic at removing the low-value stuff like memes, "me too", snarky comments, etc. As a result, the signal-to-noise ratio is much higher, and it doesn't need loads of subreddits.
There are plenty of technical and expert subreddits which police low-value comments just as well as HN. It's simply not the case that Hacker News, with its one or maybe two moderators, does a better job of moderation than every single subreddit.
